Relationship of birth weight to the phenotypic expression of Gilles de la Tourette's syndrome in monozygotic twins.

We studied 16 pairs of monozygotic twins (mean age, 12.8 +/- 1.4 years; age range, 8 to 26 years; sex, 12 male pairs, four female pairs) in whom at least one twin had Gilles de la Tourette's syndrome (TS) to determine the concordance rates for TS and tic disorders and to examine environmental factors accounting for intrapair differences in tic severity. In this cohort, the concordance rate for TS was 56%, and the concordance rate for tic disorders was 94%, supporting a primary genetic basis for TS and tic disorders with a high rate of penetrance for the gene. Thirteen of the pairs had differing birth weights and the lower birth-weight twin had a higher tic score in 12 of these pairs. The magnitude of the intrapair birth-weight difference (BWD) strongly predicted the magnitude of the intrapair tic score difference. The difference in tic severity could not be explained by any postnatal medical events. These findings suggest that crucial events affecting the phenotypic expression of TS occur in utero and that the factors causing the BWD also are related to tic severity.